The process of forming threads on the inner surfaces of holes is generally known as tapping. The tools used to form the threads are generally referred to as taps. Taps are generally classified into two categories “cut taps” and “form taps” based on the method used to produce the threads in the finished part being tapped.
A form tap creates threads on the inner surface of a hole by shaping and forming the material of the hole into the desired configuration. This process is also referred to as flowing the metal, cold forming, cold working or thread rolling. The leading tip of a form tap is narrowed or angled (chamfered) below the size (diameter) of the hole to be tapped, to allow the tap to be properly aligned with a hole, and to begin the forming process gradually when the form tap is used. The threads on the angled portion of a form tap are not ground or sheared into partial/truncated threads. Instead, the threads in the angled or chamfered portion of a form tap are generally formed as full threads having a crest and root configuration that is similar to the rest of the threads on the form tap. Traditional methods of manufacturing form taps tend to produce an error in the spacing, or pitch, of the threads between the crests of the threads of the finishing portion of the form tap and the crests of the threads of the angled or chamfered portion of the tap. This thread shaping error is generally known as lead error.
When a form tap has a lead error it may have a shorter lifespan and require increased torque to operate. One known attempt to correct lead error is the use of advanced CNC thread forming machines that allow for precise, computer control of the thread grinding process. However, form taps produced using CNC machines still tend to have a lead error between the crests of the threads on the finished form tap.
Some form taps have lobe portions around the circumference of the form tap that are separated by alternating relief portions. The lobe portions define “high points” or radial maxima of the threads at a given axial position, while the relief portions have a smaller radius than the lobe portions.
U.S. Pat. No. 4,235,149 (Veldman) describes a thread forming screw having a helical thread and a tapered lead-in section with a polygonal cross section. The screw is formed from a conventional blank by rolling the blank between a pair of thread rolling dies having means thereon to form alternating convex lobes and concave relief areas in the frusto-conical surface of the blank. Thus, the screw may be formed with a polygonally shaped threading end without a prior working of the blank's frusto-conical surface. Curved ramp surfaces smoothly interface the lobes and concavities to allow metal in a workpiece being threaded to roll into the concavities thereby reducing the torque needed to form a thread in the workpiece.
The screw described by Veldman is intended for one time use, and is designed to form threads while being screwed into an article and then remain attached to the article. To form the threads in the workpiece screw includes a plurality of lobes separated by concave relief areas and curved ramp surfaces for smoothing the transition between the lobes and the concavities. Because of its fastening function, all of the lobe and concavity features are limited to the tapered lead-in section of the screw and the fastener described by Veldman is preferably formed with a continuous helical thread having a constant inner and outer diameters from the first thread adjacent the leading end section to the last thread so that when the screw is fully threaded into the workpiece it provides a full thread grip because of the full engagement between the threads and the workpiece. Based on the teachings of Veldman, a skilled person may not consider extending the lobes and concavities beyond the tapered lead in section, past the first thread, because of the potential decrease in fastening strength.
Veldman also teaches that the majority of the region between adjacent lobes should comprise concave relief portions, that the need to have a preformed polygonal cross section on the tapered lead-in section is avoided and likewise a convex surface between the lobes is also avoided.
In contrast, form taps may be re-usable tools that are used to form threads in other articles hundreds or thousands of times before needing to be replaced.